This problem is done with the knowledge of senior two, and the inverse proportional function is learned in senior two.

Vieta Theorem is a junior high school student. Now in junior high school, Vieta Theorem has been deleted from the chapter of quadratic equation of one variable, so I don't want to learn it.

The practice used the next day is as follows:

∵K=-√3/3,∴∠ABE=∠ADF=30

∴EB=√3/2AB,∴AB=2√3/3EB

It is easy to prove that AB=CD according to congruence.

So AC=BD, BD=2BF,

∫ab×AC = 2√3/3eb×2BF =(4√3/3)×K = 4

K = root number 3